The research focus of the Unit of Molecular Virology in the Laboratory of Cell and Molecular Regulation has been the determination of the individual steps in the process of enveloped retroviral entry. Our research interests are in part based on our expectations that elucidation of retroviral entry events will guide the development of the next generation of viral vectors, permitting more efficient targeted gene delivery. Viral entry initiates with the irreversible binding of the envelope protein to a specific receptor on the target cell followed by protein-mediated fusion which allows retroviral penetration of the target cell membrane. The fusion process is not energetically favorable; viral and plasma membranes are subject to strong repulsive hydration, electrostatic and steric barriers. How viruses surmount these barriers is critical to our understanding of, not only viral entry, but membrane fusion reactions critical to neurotransmission, fertilization and membrane recycling. The final step in viral entry is internalition. The enzymatic activity of the replication enzyme, reverse transcriptase, appears to be associated with the nucleocapsid rather than as soluble proteins therefore the relatively large nucleocapsid must get through cytoskeletal cortex and be deposited more or less intact in the cytoplasm of the cell. The complex of actin filaments and actin binding proteins can be up to 100 nm in length and this complex not only has the capacity to exclude eucaryotic ribosomes and other organelles from the area immediately adjacent to the plasma membrane but also the potential to restrict the substantially larger incoming nucleocapsids from entering areas deeper in the cells. The final stage of viral entry the internalization of the nucleocapsid remains the least well characterized of the three stages of viral entry There are several advantages of working w/GALV as a model virus for studying enveloped virus entry are that it is a simple enveloped virus made up of only 3 component parts with no accessory proteins. Secondly we have developed GALV based gene transfer vectors and can use these vectors instead of replication competent virus to conduct entry experiments. Finally we have determined that the cell surface protein on human cells that functions as a GALV receptor is a type III phosphate transporter. We have recently made mutant receptors that mediate virus binding but do not allow virus-cell fusion. We have also identified receptors that facilitate wild type virus entry but do not permit entry of GALV vectors bearing mutation in their envelope protein. The availability of these reagents should allow the dissection of the envelope and receptor regions that play an important role in GALV receptor binding and GALV envelope fusion to target cell membranes.